This invention relates to high temperature superconducting devices, and, more particularly, to the adaptation of high temperature superconducting quantum interference devices for radio communication.
In underground areas, such as mines, tunnels, boreholes, and the like, there is a need to communicate between personnel, equipment, etc., including mobile communication. Solutions of the electromagnetic wave equations in a conducting medium show that the signal strength, in both electric and magnetic fields, varies as exp(-z/.delta.), where z is the distance from the surface of the conducting medium and .delta. is the penetration depth, given by .delta.=(2.rho./.mu..omega.).sup.1/2. Here, .omega. is the angular frequency, .rho. is the resistivity of the medium, and the source is a plane wave from above the surface. For typical sedimentary rocks, .rho..congruent.100 .OMEGA.-m, implying that radio frequency signals above about 500 kHz travel less than 10 meters into rock masses.
The ground penetrating signals are much larger at low frequencies, but the receivers for these low frequencies usually require a very large antenna (on either the transmit or receive side) to overcome the electrical noise of typical semiconductor components. For example, one mine paging system uses a transmit antenna several kilometers in diameter to achieve sufficient amplitude to drive a conveniently sized wirewound ferrite core receiver.
The difficulty of direct radio wave communication in underground areas has led to various attempts to communicate underground, e.g., using tunnel structures as transmission lines or using leaky coaxial cable with repeaters. But these attempts suffer from a limited range and/or are easy to interrupt. In short, a satisfactory wireless underground mobil communication system does not exist.
The critical performance parameter for communicating at these low frequencies is the noise of the receiver. Superconducting components have the lowest noise of any electronics technology and have been used in applications where an extremely low noise floor is critical, such as radio astronomy, noninvasive magnetoencephalography, or various scientific instruments. Superconductors are divided into two classes: low temperature superconductors that require liquid helium (4 Kelvin) to operate and high temperature superconductors (HTS) that require liquid nitrogen (77 Kelvin) to operate. The former requires a large cryogenic enclosure and cannot be used in a man-portable application. The latter requires cryogenic volumes no larger than a coffee cup; e.g., a 100 cubic cm of liquid nitrogen is easily sufficient for an 8 hour shift. The entire cryogenic system weight for a thin film device may then be reduced to under one kilogram.
Thus, it is an object of the present invention to provide direct mobile radio communication to underground areas.
It is another object of the present invention to provide a portable high temperature superconducting device for low noise communication.
One other object of the present invention is to provide a compact yet sensitive low frequency receiver.
Additional objects, advantages and novel features of the superconducting receiver will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.